I think U_LastOn is updated with each page load as part of the send_header function. So if you visit the site - it should get updated. It won't necessarily be the last time you were there - I think it'd be closer to the current time.
That other thread is all about "new since last visit" and how that's not really possible given how LastOn is handled.
Plus, like I said - I don't think reading them from there would necessarily mark them as read - so then you'd go to the main index and they'd all still be marked as unread. It might be quirky if you are bypassign the mechnism to mark the posts read.
Although it's not a perfect system now. I know that using the last post link often does not mark the post read. But if you are reading a post and click a link to a post in another forum.... often the whole forum will get marked as read.
So if this were tracked on a per post basis - it woudl be doable. But I dont' think threads tracks it like that. It would be much too slow on large sites.
I don't know how it could be done, but I think it would have to play off the w3t_Last table which I *think* typically keeps track of the last thread you've read in each forum and go from there.
Maybe Rick can enlighten us with more details of how this post tracking works.... or if he can figure it out.... include it in 6.4.